The Five Basic Tools of 'Paper'
I fell in love with the 'Paper' app when it was first released. It's a very stripped down, simple app. You have a simple set of 5 tools and a basic, muted color palette. That's it. The tools are in a handy 'drawer' that slides down, out of your way, to let you get down to business. You can organize your drawing in 'notebooks' that mimic Moleskine journals. You can add/delete pages and move them from one journal to the other. The best part (for me) is the ability to share drawings via email, Tumblr or Twitter, or just save to Camera Roll.
'Paper' is great for loose illustrations and diagrams. It has no layers, filters, import or color correction and you can't change the color palette. But that's kind of the point - keep the tools out of the way and get your ideas on paper. If you're looking for something more robust for the iPad, I'd suggest Procreate.